In 1966, German biologist Willi Hennig developed cladistics (kluh-DIS-tiks), a system of phylogenetic analysis that uses shared and derived characters as the only criteria for grouping taxa. A shared character is a feature that all members of a group have in common, such as hair in mammals or feathers in birds. A derived character is a feature that evolved only within the group under consideration. For example, feathers are thought to be a derived character for birds. Among living and fossil animals, the only animals that have feathers are birds and a few extinct reptiles that were very similar to birds in other ways. Therefore, it is reasonable to hypothesize that feathers evolved only within the bird lineage and were not inherited from the ancestors that birds share with reptiles.
Cladists assume that organisms that share one or more derived characters probably inherited those characters from a common ancestor. Cladists use the term clade for the group of organisms that includes an ancestor plus all of its descendants. Notice that clades do not have category names such as "class" or "phylum." Cladists create phylogenetic diagrams called cladograms (KLAD-uh-GRAMZ), such as the one in Figure 17-3.
Because cladists use strict criteria, their taxonomies may differ from those of traditional systematists. For example, traditional systematists grouped crocodiles with turtles, lizards, and snakes in the class Reptilia, but placed birds in their own class, Aves.
In contrast, cladistic taxonomists have grouped crocodiles and birds together in a clade named Archosauria. This grouping reflects the hypothesis that crocodiles and birds share a more recent ancestor than either group shares with any other animals. Archosauria is then grouped with successively less-related clades of animals.
Because cladistic analysis is comparative, the analysis deliberately includes an organism that is only distantly related to the other organisms. This organism is called an out-group. The out-group is a starting point for comparisons with the other organisms being evaluated.
To make a cladogram like Figure 17-4, start by making a data table like Table 17-2. Place organisms in a column at the left and their characteristics in a row at the top. Choose an out-group organism. In our example, moss has fewer traits in common with and seems most distantly related to the other groups in the table. So, moss is the choice for an out-group. Score the characters that are lacking in the out-group as a 0, and score the presence of a derived character as a 1. For example, mosses lack seeds, so they receive a 0 for the "seeds" character. Only pines and flowering plants have seeds, so they receive a 1 for that character.
The table now reveals the derived character that is shared by most of the taxa. In this example, that character is vascular tissue— vessels that transport water and sugars. This character is shown at the base of the first branch of the cladogram. The second most common character is seeds. Because ferns lack seeds, they are placed on the second branch of the cladogram. The least common character is flowers. Because pines lack flowers, they are placed on the third branch. Finally, flowering plants are placed on the last branch.
The resulting "tree" is a cladistic hypothesis for the evolutionary relationships among these plants. In addition to considering obvious physical characters, such as seeds and flowers, cladists may consider molecular characters, such as an individual nucleotide in a gene sequence or an amino acid sequence within a protein.
TABLE 17-2 Data Table for Cladogram
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